In a mobile cellular communication system there are no fix access points for a mobile station. Consequently, when the network of the communication system is to contact the mobile station it first has to locate it within the system. In such a case, the network typically sends a paging message intended to the mobile station, which upon reception of the paging call transmits a reply message enabling the network to locate it. A mobile station, thus, has to listen on a particular paging or control channel for any paging messages transmitted from the network. However, continuously monitoring of the paging channel is a very power consuming task, shortening the lifetime of the battery of the mobile station.
In order to save the mobile station's battery, DRX (discontinuous reception) may be introduced in the communication system. DRX makes it possible to divide the common paging channel into several paging groups or sub-channels. A mobile station then only resides and monitors a particular paging sub-channel, known locally both by the mobile station and the network. More importantly, paging messages to a specific mobile station may be scheduled and transmitted at a time derived from information of DRX parameters. Thus, the mobile station knows when relevant paging messages will be sent and can power down for the periods when no paging messages are expected, increasing the lifetime of the battery. In addition, DRX also enables the mobile station to process other tasks during the non-paging periods.
DRX has been employed in communication systems supporting GPRS (General Packet Radio System). A mobile station in RR (Radio Resource) packet idle mode listens to a paging sub-channel on a PCCCH (Packet Common Control CHannel) or a CCCH (Common Control CHannel) at some time instances (paging period) determined by its DRX parameters. These DRX parameters include SPLIT_PG_CYCLE, which firstly indicates whether the mobile station wants to use DRX or not and secondly is used to determine the correct paging period, i.e. the time instances at which paging messages may be expected for the mobile station when camping on PCCCH or, if period split is supported, on CCCH. SPLIT on CCCH indicates whether the mobile station supports the SPLIT_PG_CYCLE option on CCCH, if not supported the corresponding parameter BS_PS_MFRMS is used to determine the correct paging period as for GSM (Global System for Mobile communications). The SPLIT_PG_CYCLE parameter is determined by the mobile station itself among about one hundred different values and is transmitted together with the other DRX parameters to the network in GPRS attach request and routing area update request messages. For a mobile station monitoring a sub-channel of CCCH where SPLIT_PG_CYCLE is not supported or for a mobile station in a GSM network system, the BSS (Base Station Subsystem) broadcasts the corresponding BS_PS_MFRMS parameter. For more information on DRX and paging in GSM and GPRS, reference is made to [1–4]
DRX has also been employed for UTRAN (UMTS Terrestrial Radio Access Network; UMTS (Universal Mobile Telecommunications System)) for mobile stations in RRC (Radio Resource Control) idle mode and some operational states of the RRC connected mode in order to reduce power consumption. For determining its paging occasions, the mobile station calculates a DRX cycle length based on a DRX cycle length coefficient. The mobile station receives information of a CS (Circuit-Switched) CN (Core Network) specific DRX cycle length coefficient in system information from the CS CN domain. A corresponding PS (Packet-Switched) CN specific DRX cycle length coefficient is negotiated between the mobile station and the PS CN domain. Correspondingly, UTRAN may provide a UTRAN DRX cycle length coefficient included in e.g. physical channel reconfiguration and update confirm messages to the mobile station. The mobile station then calculates a UTRAN DRX cycle length and PS and/or CS CN specific DRX cycle lengths based on the provided coefficients and uses the shortest cycle length for determining its paging occasions. For UTRAN there are only four allowed values of the CN specific DRX cycle length coefficients and eight possible UTRAN DRX cycle length coefficients, respectively. More information on paging and DRX in UMTS/UTRAN is provided by [5–7]
For a hybrid cellular communication system consisting of network elements, i.e. core network and radio access network, of two different systems, such as a GSM RAN and a UMTS CN, using different paging principles and techniques, there are problems in obtaining an efficient solution supporting DRX. This is the situation for GERAN (GSM/EDGE Radio Access Network, EDGE (Enhanced Data rates for Global Evolution)) operating in Iu mode, where a proposed solution is that GERAN broadcasts a default SPLIT_PG_CYCLE for use by a mobile station in RRC idle mode. However, this solution results in some major disadvantageous for the mobile station. Firstly, the mobile station will have no possibility of affecting the time length of the paging period and, secondly, it has to support all values of the default SPLIT_PG_CYCLE broadcast by GERAN, which may be as many as for GPRS, i.e. about one hundred: